1. Field of the Invention
The present invention relates to a control circuit power supply circuit to be incorporated into a power source circuit in which a voltage outputted from a transformer is rectified and smoothed by the switch on-off operation of a switch element and operative to supply power to a control circuit for controlling the switch on-off operation of the switch element.
The present invention also relates to a power supply circuit comprising the control power supply circuit.
2. Description of the Related Art
FIG. 9 shows a prior art control circuit power supply circuit in the state where it is incorporated in a resonance reset forward converter as a power source circuit. The power source circuit shown in FIG. 9 comprises a direct current input power source 1, a transformer 2, a choke input rectifying circuit 3, a switch element SW, a control circuit 4, and a control power supply circuit 5. The choke input rectifying circuit 3 comprises diodes D1, D2, a choke coil L, and a smoothing capacitor C. The control circuit 4 has a circuit configuration in which a pulse signal for controlling the on-off state of the switch element SW (MOS-FET in the circuit shown in FIG. 9) is applied on the gate of the switch element SW to control the switching of the switch element SW. The control circuit power supply circuit 5 comprises resistors 6,7, a transistor element 8 (MOS-FET in the circuit shown in FIG. 9), a Zener diode 10, a power source capacitor 11, a choke coil 12, and diodes 13, 14.
As shown in FIG. 9, the positive electrode side of the input power source 1 is connected to one side of a primary coil N1 of the transformer 2. The other side of the primary coil N1 is connected to the drain of a transistor element as the switch element SW. The source of the switch element SW is connected to the negative electrode side of the input power source 1. Further, the gate of the switch element SW is connected to the control circuit 4.
One side of a secondary coil N2 of the transformer 2 is connected to the cathode of the diode D1. The anode of the diode D1 is connected to the anode of the diode D2. The cathode of the diode D2 is connected to the other side of the secondary coil N2.
Further, a node between the diodes D1, D2 is connected to one side of the choke coil L. The other side of the choke coil L is connected to one side of the smoothing capacitor C. The other end of the smoothing capacitor C is connected to a node between the secondary coil N2 and the diode D2. A load 15 is connected in parallel with the smoothing capacitor C.
The transformer 2 is provided with an auxiliary coil N3. One side of the auxiliary coil N3 is connected to the cathode of the diode 14. The anode of the diode 14 is connected to the anode of the diode 13. The cathode of the diode 13 is connected to the other side of the auxiliary coil N3. One side of the choke coil 12 is connected to a node between the anode of the diode 13 and the anode of the diode 14.
The other side of the choke coil 12 is connected to the negative electrode side of the input power source 1, the anode of the Zener diode 10, and one side of the power source capacitor 11, respectively. The other side of the power source capacitor 11 is connected to the source of the transistor 8, the cathode of the diode 13, and the control circuit 4, respectively.
The cathode of the Zener diode 10 is connected to the gate of the transistor element 8 and one side of the resistor 7, respectively. The other side of the resistor 7 is connected to the positive electrode side of the input power source 1. The drain of the transistor element 8 is connected to one side of the resistor 6. The other side of the resistor 6 is connected to the positive electrode side of the input power source 1.
The power source circuit shown in FIG. 9 is configured as described above. As well known, when the switch element SW switches on under the switching control of the control circuit 4, the power of the input power source 1 energizes the electric path from the positive electrode side of the input power source 1 through the primary coil Nl and the switch element SW to the negative electrode side of the input power source 1, to provide power for the transformer 2. The power outputted from the secondary coil N2, in dependence on the power supplied to the transformer 2 in the above manner, is rectified and smoothed by the choke input rectifying circuit 3 to be supplied to the load 15.
The control circuit 4 so controls the pulse width t of a pulse signal as shown by waveform B in FIG. 5 to be applied on the switch element SW, that the output voltage Vout from the power source circuit to the load 15 equals a predetermined voltage Vst. More particularly, the pulse signal rises every predetermined period T, when the output voltage Vout becomes lower than the setting voltage Vst, the shortage of the output voltage Vout on a setting voltage Vst basis is compensated by lengthening the pulse width t of the pulse signal (that is, the ratio (t/T (duty)) of the pulse width t to the period T is increased) to increase the electric energy to be supplied from the input power source 1 to the transformer 2 to increase the output voltage from the transformer 2. On the contrary, when the output voltage Vout becomes higher than the setting voltage Vst, the excess of the output voltage Vout over the setting voltage Vst is compensated for the stabilization control of the output voltage Vout by shortening the pulse width t of the pulse signal (the duty is reduced) to decrease the voltage output from the transformer 2.
The control circuit power supply circuit 5 is so configured as to supply power having the predetermined control voltage Vse to the control circuit 4 whereby the control circuit 4 can carry out the switching control of the switch element SW with high stability. A main power supply circuit is provided therein in which a power outputted from the auxiliary coil N3 is rectified and smoothed by the power source capacitor 11, the choke coil 12, and the diodes 13, 14 in the control power supply circuit 5, and supplied to the control circuit 4, and a start-up circuit is provided, in which a power having the control voltage Vse, provided through the resistor 6 and the transistor 8 in the control circuit power supply circuit 5, by using power from the input power source 1, is supplied to the control circuit 4.
The auxiliary coil N3, in the normal condition, outputs a power having a stable voltage by the switching operation of the switch element SW. Accordingly, a power having the control voltage Vse can be stably supplied from the main power to the control circuit 4 in the normal condition, by setting the circuit constants of the respective elements of the control power supply circuit 5 so that the setting control voltage Vse can be stably supplied to the control circuit 4 in the normal condition.
As the Zener diode 10, one which can apply a voltage nearly equal to the control voltage to the gate of the transistor element 8 is employed, and thereby, when a voltage nearly equal to the control voltage Vse is supplied from the main power supply circuit to the control circuit 4 as described above, the transistor 8 is controlled by the Zener diode 10 to be in its conduction-off state so that the supply of power from the start-up circuit to the control circuit 4 is prevented.
At starting up, or if an abnormality causes an excess voltage to be applied to the load 15 and the smoothing capacitor C (when the overvoltage protection is operated), the switch element SW is brought into its down state, which prevents the auxiliary coil N3 from outputting power. In this case, the transistor element 8 gets into the conducting state by the control of the Zener diode 10, and a power of the input power source 1, passing the resistor 6 and the transistor element 8 sequentially, is supplied to the control circuit 4.
In this case, the transistor element 8 acts as a series regulator. On the gate of the transistor element 8, a predetermined control terminal voltage for supplying a power at the control voltage Vse from the transistor element 8 to the control circuit 4 is stably applied by the Zener diode as described above, so that the power at the control voltage Vse can be stably supplied to the control circuit 4.
At a low load, that is, when the load 15 has a high resistance to reduce significantly the current to be supplied to the load 15, or when a current flowing in the choke input rectifying circuit 3 becomes excessively large in event of an abnormality (when the overcurrent protection is operated), a pulse signal having a very low duty is applied from the control circuit 4 to the switch element SW. This causes the situation that a voltage to be supplied from the main power supply circuit to the control circuit 4 becomes much lower than the setting control voltage Vse.
In this case, by the circuit configuration in FIG. 9, the transistor element 8 is controlled by the Zener diode 10 to be brought into its conducting state, and thereby, a power at the control voltage Vse can be supplied from the input power source 1, passing the transistor element 8, to the control circuit 4.
However, as the conducting time period of the transistor element 8 as the series regulator is longer and the difference between the voltages of the transistor element 8 on the incoming, outgoing sides is larger, so the conduction loss in the transistor element 8 is greater. ordinarily, the continuation time period of the low load or overcurrent state is long, and the voltage of the input power source 1 is much higher than the control voltage Vse to be supplied to the control circuit 4, and the difference between the voltages of the transistor element 8 on the incoming, outgoing sides is large. Thus, there is a problem that the conduction loss in the transistor element 8 is great and the circuit efficiency is low at the low load or during overcurrent protection operation.
Further, heat stress in dependence on the conduction loss is applied to the transistor element 8. Accordingly, at a light load and overcurrent protection, a great heat stress is imposed on the transistor element 8. For this reason, as the transistor element 8, one having a high heat durability is employed, allowing for the generation of a large heat stress at a low load and overcurrent protection operation. However, devices having such a high heat durability are difficult to be miniaturized, and are bulky. There arises a problem that the package of the power source circuit is large in size.